Welcome to the Event, @ianleonard!
Here is a great resource from the IRS about documentation of business expenses:
https://www.irs.gov/businesses/small-businesses-self-employed/what-kind-of-records-should-i-keep#:~:...
Receipts, account statements, invoices are all good methods.
You will use these when you are figuring out your Profit and Loss Statement for your business at tax time.
The IRS may ask for supporting documentation at any time - whether for an audit or just informational.
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